Technical Field
The present disclosure relates to an electric wire with terminal in which an aluminum alloy wire rod is used.
Background
In the related art, an electric wire with terminal is used as an electric wiring structure for transportation vehicles such as automobiles, trains, and aircrafts, or an electric wiring structure for industrial robots. The electric wire with terminal is a member including an electric wire having a conductor made of copper or copper alloy and fitted with a terminal (connector) made of copper or copper alloy (e.g., brass).
With recent rapid advancements in performances and functions of automobiles, various electrical devices and control devices installed in vehicles tend to increase in number and electric wiring structures used for devices also tends to increase in number. On the other hand, for environmental friendliness, lightweighting is strongly desired to improve fuel efficiency of transportation vehicles such as automobiles.
As one of the measures for achieving recent lightweighting of transportation vehicles, there have been, for example, studies in changing the conductor of an electric wiring structure from the conventionally used copper or a copper alloy into aluminum or an aluminum alloy which is more lightweight. Since aluminum has a specific gravity of about one-third of a specific gravity of copper and has a conductivity of about two-thirds of a conductivity of copper (in a case where pure copper is a standard for 100% IACS, pure aluminum has approximately 66% IACS), a pure aluminum conductor wire rod needs to have a cross sectional area of approximately 1.5 times greater than that of a pure copper conductor wire rod to allow the same electric current as the electric current flowing through the pure copper conductor wire rod to flow through the pure aluminum conductor wire rod. Even an aluminum conductor wire rod having an increased cross section as described above is used, using an aluminum conductor wire rod is advantageous from the viewpoint of lightweighting, since an aluminum conductor wire rod has a mass of about half the mass of a pure copper conductor wire rod. Note that, “% IACS” represents a conductivity when a resistivity 1.7241×10−8 Ωm of International Annealed Copper Standard is taken as 100% IACS.
However, it is known that pure aluminum, typically an aluminum alloy for transmission lines (JIS (Japanese Industrial Standard) A1060 and A1070), is generally poor in its durability to tension, resistance to impact, and bending characteristics. Therefore, for example, it cannot withstand a load abruptly applied by an operator or an industrial device while being installed to a car body, a tension at a crimp portion of a connecting portion between an electric wire and a terminal, and a cyclic stress loaded at a bending portion such as a door portion. On the other hand, an alloyed material containing various additive elements added thereto is capable of achieving an increased tensile strength, but a conductivity may decrease due to a solution phenomenon of the additive elements into aluminum, and because of excessive intermetallic compounds formed in aluminum, a wire break due to the intermetallic compounds may occur during wire drawing. Therefore, it is essential to limit or select additive elements to provide a sufficient elongation property to prevent a wire break, and it is further necessary to improve impact resistance and bending characteristics while ensuring a conductivity and a tensile strength equivalent to those in the related art.
For example, Japanese Laid-Open Patent Publication No. 2012-229485 discloses a typical aluminum conductor used for an electric wiring structure of the transportation vehicle. This is an extra fine wire that can provide an aluminum alloy wire rod and an aluminum alloy stranded wire having a high strength and a high conductivity, as well as an improved elongation. Also, Japanese Laid-Open Patent Publication No. 2012-229485 discloses that elongation is sufficient, which results in improved bending characteristics. However, for example, it is neither disclosed nor suggested to use an aluminum alloy wire as a wire harness attached to a door portion, and there is no disclosure or suggestion about bending fatigue resistance under an operating environment in which a high cycle fatigue fracture is likely to occur due to repeated bending stresses applied by opening and closing of the door.
Recently, it is being recognized that, when manufacturing an aluminum alloy wire rod used for automobiles, particularly an aluminum alloy wire rod of about φ 0.1 mm to φ 1.5 mm, the following three problems arise. The first problem is that, when used for a repeatedly bent section such as a door section of an automobile as described above, a high bending fatigue resistance is required. Since aluminum has a poor bending fatigue characteristic as compared to that of copper, an applicable place is limited. The second problem is that, because of a high proof stress, a large force is required when attaching a wire harness, and thus a working efficiency is low. The third problem is that, because of a low elongation property, it cannot withstand an impact while installing the wire harness or after installing the wire harness, and thus a wire break or a crack occurs. In order to solve all of these problems, provided that conductivity is high, an aluminum alloy wire having a high bending fatigue resistance as well as an appropriate proof stress and a high elongation property is necessary.
As aluminum alloys having both high strength and high conductivity, alloy in which Mg and Si, Cu, Mn or the like are added are known. For example, according to Japanese Patent No. 5155464, these elements are added to achieve a tensile strength of greater than or equal to 150 MPa and a conductivity of greater than or equal to 40%. Further, according to Japanese Patent No. 5155464, by manufacturing a wire rod having a maximum grain size of less than or equal to rod 50 μm, an elongation property of greater than or equal to 5% is achieved at the same time.
However, the aluminum alloy wire rod according to Japanese Patent No. 5155464 is not capable of achieving both high conductivity and a high elongation property as well as both high bending fatigue resistance and an appropriate proof stress, and thus the above-mentioned three problems cannot be solved simultaneously.
An automotive wire harness or the like generally employs an electric wire with terminal including a crimp terminal of copper or a copper alloy fitted at an end portion of a coated wire having a wire rod of a copper alloy conductor as a base, but when the above mentioned wire rod is replaced with an aluminum alloy, a problem of corrosion due to a potential difference arises.
In this regards, recently, there is a development in a technique for solving the problem of corrosion. Using a terminal including a one end closed barrel portion, a connecting portion between the wire rod and the terminal is formed in the barrel portion, and the barrel portion is crimped so that moisture does not enter into the barrel portion. However, conventionally, a relatively soft material such as pure aluminum is used as a wire rod, and when crimping an electric wire (i.e., when a crimp force is applied from an outer peripheral portion of the wire rod), the wire rod of such a material tends to extend in a longitudinal direction and escape, rather than producing deformation that is repulsive in a plane perpendicular to the longitudinal direction.
Therefore, it was not possible to suppress a void fraction in the barrel portion at a low level, and water was likely to enter inside. Accordingly, in a case of a copper terminal, it was a cause of corrosion between dissimilar metals.
Also, inside the one end closed tubular barrel portion, a tip of the aluminum alloy wire rod abuts an inner wall surface of the barrel portion at a leading end side. Accordingly, there may be cases where a desired crimping property and a water-proof property cannot be obtained by the barrel portion. For example, a part of the barrel portion that is weak in strength may break, or an entire electric wire may be pushed back towards a rear end side with respect to the terminal, and an aluminum alloy wire rod without a coating may be exposed from an opening portion of the barrel. Further, crimping of a resin portion of the electric wire may be insufficient, which may decrease a pull out strength.
In order to prevent this, for example, a space elongated in a longitudinal direction may be provided inside the barrel portion to take into account a possible elongation amount of the aluminum alloy wire rod. However, in such a case, the entire terminal becomes excessively long in the longitudinal direction.
Here, a connector housing provided on a wire harness is designed to have a shape, size, etc., assuming that a terminal comprising copper or a copper alloy is to be inserted. Therefore, in order to accommodate a terminal in a housing, it is necessary to place, particularly, a longitudinal length of the terminal within a predetermined range. However, in a case where the barrel portion as described above becomes excessively long in a longitudinal direction, there was a problem that a rear end of the terminal projects from the connector housing.
The present disclosure is related to providing an electric wire with terminal including a terminal having a one-end closed barrel portion and an electric wire including an aluminum alloy wire rod, in which the aluminum alloy wire rod has, while maintaining an elongation property and conductivity that are equivalent or greater than those of conventional products, both an appropriate proof stress and a high bending fatigue resistance, and in which moisture is less likely to enter inside the barrel portion, and also the terminal can be configured to have a compact structure in the longitudinal direction.
The inventors have found that, when an aluminum alloy wire rod is bent, a stress produced in an outer peripheral portion of a conductor is greater than a stress produced at a central portion, and a crack was likely to occur in an outer peripheral surface. Accordingly, the inventors have focused on a case in which, when an aluminum alloy has a small crystal grain size, a crack encounters a grain boundary for an increased number of times and a propagation speed decreases, and carried out assiduous studies. As a result, the inventors have reached the findings that, with an average crystal grain size at the outer peripheral part of the aluminum alloy wire rod being a value within a predetermined range, while maintaining a high conductive property, a bending fatigue resistance is improved, and further, an appropriate proof stress and a high elongation property are obtained.
Still further, the inventors have found that, under a crimping force from an outer periphery, an aluminum alloy wire rod as described above does not escape in a longitudinal direction like pure aluminum, but rather deforms isotropically. This implies that, when crimped at the barrel portion of the terminal, an aluminum alloy wire rod as described above repulses isotropically in a cross section subjected to the crimping force, in other words, less likely to escape in a longitudinal direction.
From the foregoing studies, the inventors have found that, the aforementioned aluminum alloy wire rod is, when in combination with a terminal having a one-end closed tubular barrel portion, an aluminum alloy wire rod suitable for controlling an elongation of the electric wire in a predetermined range when crimping the barrel portion, and thus an electric wire with terminal suitable for an automotive wire harness can be obtained.